This invention relates to a method of removing mercury species, in particular, elemental mercury, from exhaust gases of industrial and combustion processes.
Exposure to high levels of mercury is associated with serious neurological and developmental effects in human beings. Concentrations of mercury in the air are usually low and of little concern, but once mercury enters water, it can accumulate in fish and cause harm to people who eat mercury-contaminated fish. Many industrial and combustion processes produce exhaust gases having low, but still significant, levels of mercury. For example, even if the levels of mercury in coals are low, mercury emissions from coal-fired power plants have recently been determined to pose a significant hazard to public health. Thus, the reduction of mercury in the exhaust gases of industrial and combustion processes is of great importance.
It is known that exhaust gases emanating from industrial and combustion processes may contain mercury in elemental, oxidized, and particulate forms. Elemental mercury in exhaust gases does not stick to soot and other particles entrained with the gases, but tends to remain in vapor form even after the exhaust gases are cooled to about 65° C. Therefore, at least a portion of elemental mercury in the exhaust gases is not recovered by conventional dust removal devices, such as electrostatic precipitators, fabric filters, or conventional scrubbers, but is instead released into the atmosphere.
Volatilized mercury from industrial and combustion processes may be recovered by adsorption on powdered activated carbon injected into the exhaust gases upstream of air pollution control devices. However, mercury adsorption on activated carbon is not very efficient. Activated carbon treated by sulfur compounds is more efficient but cannot be regenerated after it is saturated. Therefore, the use of activated carbon for mercury removal tends to produce large amounts of unregenerated adsorbents mixed with fly ash from the process, and thus, leads to problems involved in disposing large amounts of mercury-containing wastes.
Zeolites are widely used as adsorbents, catalyst carriers, and filters. These compounds are framework silicates that contain aluminum as a substitute for a portion of the silicon atoms. Such a substitution results in a charge imbalance which is compensated for by the addition of an alkali or alkaline ion, such as sodium. This alkali or alkaline ion may be exchanged with other ions to produce different species of zeolites. Zeolite frameworks are arranged so that the crystal structure has cavities and pores on an atomic scale. Each species of zeolite has a specific composition and structure. Therefore, each species contains uniform pores of a specific size. Zeolites have a range of physical and chemical properties because of differences in their composition and structure.
German Patent Publication No. DE 2,841,565 discloses a process for mercury adsorption, wherein zeolites, such as so-called Zeolites A, X and Y, are ion-exchanged with transition metals, such as silver, copper and zinc, prior to being contacted with a mercury-containing gas. U.S. Pat. No. 4,101,631 describes a process in which mercury vapor is selectively chemisorbed from gas streams by contact with zeolitic molecular sieves having elemental sulfur loaded thereon.
U.S. Pat. No. 4,892,567 describes a process in which mercury and water are simultaneously removed from fluids by contacting the fluids with zeolite A activated by silver or gold. U.S. Pat. No. 5,460,643 discloses a process where waste gas containing mercury is conveyed to an adsorption tower filled with ceramic particles, such as zeolite, impregnated with silver nitrate. The ceramic particles are regenerated for recycling by heating them to a temperature from about 600 to about 900° C. and converting the silver formed during the heating operation back to silver nitrate by contact with nitric acid.
U.S. Pat. No. 5,659,110 describes a process for purifying exhaust gases produced by combusting waste materials which includes reacting the gases with a mixture containing naturally occurring zeolites to remove mercury and mercury compounds from the exhaust gases. The mixture contains 10 to 20% by weight mordenite and 60 to 70% clinoptilolite.
All the methods discussed in the patents referred to above may suffer from insufficient mercury removal efficiency or incomplete regeneration. Thus, there is need for a new method that provides efficient mercury removal using an adsorbent that can be regenerated by a simple and an efficient method.